In a prior method of producing interior material of corrugated cardboard, corrugated cardboard base material of a liner and a core with a thermally melting adhesive therebetween, and being at room temperature, is placed in a thermally forming die having a high temperature of 120.degree. C. to 140.degree. C., for example, to be pressed into a predetermined shape while the thermally melting adhesive is melted to bond the liner and the core to each other in the new shape. Afterward a covering sheet is disposed having a thermally melting sheet faced to the corrugated cardboard base material, they are pressed in the thermally forming die to melt the thermally melting sheet by the high temperature of the thermally forming die to bond the covering sheet to the corrugated cardboard base material. However, the prior art has many drawbacks. Firstly, since the corrugated cardboard base material being at room temperature is placed in the thermally forming die, it is possibly pressed before it is fully plasticized, because all of the flat cardboard does not contact the curved die. This causes the corrugated cardboard base material to have wrinkles and cracks produced. Secondly, since the thermally forming die is kept at the high temperature after the thermally melting sheet is melted, the shaped corrugated cardboard base material is possibly bent or deformed due to back spring thereof when it is removed out of the thermally forming die, and the liner and the core possibly are stripped off or partially separated. This causes the product to have a lower quality. Furthermore, since the corrugated cardboard base material is deformed under the high temperature of 120.degree. C. to 140.degree. C., for example, heat of vaporization is lost to lower the temperature of the corrugated cardboard base material. This requires heating it for a longer time and also cooling it after shaping it for a longer time. Thus, working time disadvantageously becomes longer. Finally, since the thermally forming die is used, the covering sheet has to be of heat-resistant material. This means that materials of the covering sheet are limited and also that the loss of heat becomes larger.